Re: dash harness update ?
A power driver seat was an option in 68 Firebird; I have never seen a Firebird fuse box, but I assume it's similar to what Jim described for Camaro power windows, which is a standalone harness that just plugs into one of the "ACC" terminals on the fuse box for power feed. American Autowire makes the Firebird harnesses as part of their Factory Fit line, so if you gave them a call (ask for Donny Bock) they should be able to give more info.
Depending on the condition of the rest of your harnesses, and plans for the rest of the car, the 67-68 Classic Update kit from AAW might interest you. New wiring front to rear, circuits for accessories like PW, power seats, engine computers, electric fans, electric fuel pumps, etc., reasonable price, some assembly required. Very nice kit.
69 'vert project big block/TKO 600 RR
68 'vert driver RS clone -- gone!